When the menu is not open, Minecraft is recreating textures every single frame and/or tick, which causes immense performance penalties when the textures are high res like 128x Sphax.
Edit: I rewrote this hopefully explaining this a bit more clearly.
This is a Major issue when using High Resolution Texture Packs and might even have impact on even on 16x though it's a bit hard to tell. Try a 128x or 256x for large impact testing. Pretty much go into the game with a 128x texture pack and see if your fps is cut 10 times more or even in half. At this point hit esc and see if the fps goes higher. This where it seems to be a issue with glTexImage2D.
There seems to be a major issue with Rendering on the screen when it comes to glTexImage2D. I attached a profiling of the 256 texture and it seems when not in the menu it's eating CPU time like crazy. My guess would be that it's refreshing some the textures or maybe all of them ever frame. When you escape and go into the Menu this seems to solve the issue.
I do realize that when you are in the menu a few the ticks and such do shutdown. But the texture pack is clearly still there and loaded as well the chunks are still loading. Yet you go from 1 fps to 100 fps.
For my testing I get the following results:
128x 10x fps drop
256 50-100x fps drop
Vvideo Example: http://www.youtube.com/watch?v=NVP5P4E3BJA
NPS File: https://dl.dropbox.com/u/84010144/1365052614347.nps
Crash Log Asked by Mod: https://dl.dropbox.com/u/84010144/crash_report.txt